Do start with a helmet-But make certain it fits. The shop will have plenty of models and sizes for you to try so take advantage of that. Goggles and gloves next- Gloves do help and cycling specific goggles are better than sunglasses but you can get by without them. Shorts are a must- If you don't like lycra then go for the Mountain biking shorts. Apart from that nothing essential and even the shorts you can get by without, but don't wear long trousers. A "Wedge" saddle bag next and in it you want a spare tube, tyre levers and a patch kit- possibly a multi tool but you can carry loose tools to get by with. On the bike a Pump and make certain it is good enough to pump a tyre up in 2 minutes and not eventually. Then water bottles. If you don't get one on the bike- get the LBS to throw one in and a carrier aswell.

Then later on comes the "essentials" of 2nd water bottle- shortsleeve shirt, long sleeve shirt- 2nd pair of shorts, waterproff top, showerproof top, windproof top, leggings, clipless pedals and shoes and a friendly bank manager to pay for it all.